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mm/hugetlb_cgroup: call page_counter_set_max() outside VM_BUG_ON()

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hugetlb_cgroup_css_alloc() rounds the counter limit down to a multiple of
the huge page size and then applies it inside an assertion:

	VM_BUG_ON(page_counter_set_max(fault, limit));
	VM_BUG_ON(page_counter_set_max(rsvd, limit));

With CONFIG_DEBUG_VM=n, VM_BUG_ON(cond) is BUILD_BUG_ON_INVALID(cond),
i.e.  ((void)(sizeof((__force long)(cond)))), whose operand is never
evaluated.  page_counter_set_max() is not a predicate - it performs
xchg(&counter->max, nr_pages) - so on every non-debug kernel the limit is
never applied and the counters keep page_counter_init()'s
PAGE_COUNTER_MAX.

That is user-visible, because hugetlb_cgroup_read_u64_max() recomputes the
same rounded value and uses equality as its "unlimited" sentinel. 
PAGE_COUNTER_MAX is LONG_MAX / PAGE_SIZE = 2251799813685247, which is odd,
so round_down() really does change it and the two sides disagree.  With
CONFIG_DEBUG_VM=n:

	$ cat /sys/fs/cgroup/t/hugetlb.2MB.max
	9223372036854771712

and with this patch:

	$ cat /sys/fs/cgroup/t/hugetlb.2MB.max
	max

A debug option should not change cgroup output.

Call the function, then assert the result, as v6.12 did.  Use
VM_WARN_ON_ONCE() rather than restoring VM_BUG_ON(): the two are identical
under CONFIG_DEBUG_VM=n, and checkpatch asks that new code not use BUG()
variants.

Link: https://lore.kernel.org/20260817103433.191266-1-njilav@gmail.com
Fixes: 0e2759afca ("page_counter: track failcnt only for legacy cgroups")
Signed-off-by: Narek Jilavyan <njilav@gmail.com>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Cc: David Hildenbrand <david@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Narek Jilavyan 2026-08-17 10:34:33 +00:00 committed by Andrew Morton
parent a3417097fb
commit eedc8474d4
1 changed files with 5 additions and 2 deletions
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7
mm/hugetlb_cgroup.c
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@ -97,6 +97,7 @@ static void hugetlb_cgroup_init(struct hugetlb_cgroup *h_cgroup,
struct page_counter *fault, *fault_parent = NULL;
struct page_counter *rsvd, *rsvd_parent = NULL;
unsigned long limit;
int ret;
if (parent_h_cgroup) {
fault_parent = hugetlb_cgroup_counter_from_cgroup(
@ -118,8 +119,10 @@ static void hugetlb_cgroup_init(struct hugetlb_cgroup *h_cgroup,
limit = round_down(PAGE_COUNTER_MAX,
pages_per_huge_page(&hstates[idx]));
VM_BUG_ON(page_counter_set_max(fault, limit));
VM_BUG_ON(page_counter_set_max(rsvd, limit));
ret = page_counter_set_max(fault, limit);
VM_WARN_ON_ONCE(ret);
ret = page_counter_set_max(rsvd, limit);
VM_WARN_ON_ONCE(ret);
}
}